For my research in the field of orbital maneuvers I consider the orbital transfers with space tugs. For the high-thrust maneuvers I used the parameters of Fregat and Volga space tugs. However, for low-thrust transfers I can't find any example of a space tug.
What are the realistic parameters of a low-thrust space tug (dry mass, fuel mass, specific impulse, etc.)?
Links and references would be appreciated.